Inverter with variable impedance delay element

Abstract: According to the present invention, by setting the logic state of one or more delay signals to appropriate values, the resistive value of a plurality of power supply delay elements throughout an integrated circuit having distributed circuit blocks may be modified to produce desired delay times or pulse width adjustments throughout the integrated circuit. Setting delay signals to desired logic states may be accomplished by a variety of means including forcing test pads to a logic level, blowing fuses, or entering into a test mode.

Description: The present invention relates generally to integrated circuit devices, and more specifically to variable impedance power supply delay elements for use in such integrated circuit devices. Throughout the design, test, and manufacture of integrated circuit devices, such as memory devices, there exists the need to determine circuit performance. Especially during circuit debug, speed optimization, and yield optimization, certain device parameters and corresponding critical signals must be evaluated. Such critical signals may include sense amp timing, set-up and hold times, rise and falls times, etc. Evaluation of critical signals is often accomplished by introducing delays to the critical signals to change the timing of these critical signals and then measuring how the circuit responds to such delays. Introduction of delays is often controlled by delay elements. It is often necessary to distribute delay elements throughout the integrated circuit in order to effect a global delay for a critical signal, since the critical signal may be distributed across a chip or within repeated circuit blocks. For example, sense amp timing delays are often times found in each memory block. It is common for sense amp timing delays to be introduced to 8, 16, 32, or more circuit blocks. There are several methods currently used in the art to introduce delays in critical signal timing. Adjustments in the delay of critical signals may be accomplished using experimental masks, but this is costly and time consuming. It could be done using a focused ion beam (FIB), but when a number of circuit blocks must be adjusted, this can be quite time consuming, prone to error, and expensive. FIB is best suited for adjusting the circuitry at a limited number of locations, rather than repetitive operations. Also, FIB does not allow for production adjustment by blowing fuses and is therefore not ideal for a production environment. Additionally, placing fuses in each delay circuit takes a great deal of layout area and is cumbersome to layout in the central block control area of the chip. To date, methods for introducing delays to critical integrated circuit signals have required the placement of delay elements throughout the integrated circuit in distributed circuit blocks. The distribution of delay elements throughout the integrated circuit means that adjustments to these delay elements must be made at multiple locations. Methods currently used to do this, as described above, may be time consuming, prone to error, expensive, and not desirable for a production device. WO-A-92/16051 discloses a delay circuit in which the gates of two P transistors and of two N transistors are coupled to an input signal, the channels of the P transistors being connected in series, the channels of the N transistors being connected in series and a common node between the P and N transistors providing an output. One or more third P transistors parallel an uppermost one of the two P transistors and one or more further N transistors parallel the lower most of the N transistors and the gates of these additional transistors are controlled by complementary inputs to provide a variable delay. This application also envisages serially connecting plural delay circuits to provide longer delays, or multiple delays. JP-A-61052022 discloses a delay circuit for a plurality of digital control signals. A CMOS inverter is connected to respective power supplies via respective parallel-connected transistors, the gate of each transistor being controllable by a separate input. By selectively switching the gates of the FETs on, the operating current of the inverter circuit is varied and plural delay times are set. JP-A-2290328 discloses a buffer circuit comprising plural logic elements, the plural logic elements receiving a single global control signal. When the control signal is at one level, the output level of each logic circuit changes state rapidly in response to an input change, and when the control signal is at another level, the output of the logic circuits changes state slowly in response to the input change. It is therefore an object of the present invention to globally adjust the delay of critical signals or pulse widths throughout an integrated circuit device having distributed circuit blocks. It is therefore further an object of the present invention to adjust the delay of such signals in such a manner so as to be layout efficient. It is therefore further an object of the present invention to adjust the delay of such signals in such a manner so as to be transparent to the normal operating mode of the integrated circuit. Therefore, by setting the logic state of one or more delay signals to appropriate values, the resistive value of a plurality of power supply delay elements throughout an integrated circuit having distributed circuit blocks may be modified to produce desired delay times or pulse width adjustments throughout the integrated circuit. Setting delay signals to desired logic states may be accomplished by a variety of means including forcing test pads to a logic level, blowing fuses, or entering into a test mode. According to the present invention there is provided circuitry for selectively introducing delays to a signal in an integrated circuit as defined in claim 1. The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: Figure 1 is an inverter of a first type commonly used in the switching elements of integrated circuits, according to the prior art; Figure 2 is inverter circuitry which allows the impedance of power supply delay elements to be modified; Figure 3 is inverter circuitry of a second type commonly used in switching elements of integrated circuits according to the prior art; Figure 4 is inverter circuitry which allows the impedance of power supply delay elements to be modified, according to a preferred embodiment of the invention; Figure 5 is ETD circuitry, according to the prior art; Figure 6 is ETD circuitry which employs an adjustable delay block to determine ETD pulse width; and Figure 7 is ETD circuitry which employs pass gate circuitry to determine ETD pulse width. It is necessary to have the capability to introduce delays in the timing of critical integrated circuit signals or to alter pulse widths, such that these signals or pulse widths may be evaluated at various stages of design, testing, and manufacturing. In the past, this has been accomplished by placing delay elements in multiple distributed circuit blocks. However, there are difficulties with this approach as outlined in the Description of the Prior Art above. Referring to Figure 1 , an inverter 10 of a first type commonly used in the switching elements for integrated circuits according to the prior art is shown. Inverter 10 comprises a p-channel transistor 12, which is connected to power supply V CC , and an n-channel transistor 14, which is connected to power supply V SS , as shown. Input signal 16 is inverted by inverter 10 which generates output signal 18. Inverter 10 may be replaced by a variety of other logic gates such as a NAND gate or a NOR gate. According to the present invention, modification of inverter 10 allows for the impedance, i.e. the resistance, of the power supplies of the switching elements of an integrated circuit to be modulated, thereby modulating the amount of delay introduced. Referring to Figure 2 which is not part of the present invention, inverter 10 of Figure 1 has been replaced with inverter circuitry 20 which allows the impedance of power supplies V CC and V SS to be modified. Input signal 22 is inverted by inverter circuitry 20 which generates output signal 24. In the positive voltage portion of inverter circuitry 20, P-channel transistor 30 is connected in series with p-channel transistor 26, the gate of transistor 30 is connected to the V SS power supply, and transistors 26 and 30 are connected to the V CC power supply. P-channel transistor 32 and p-channel transistor 34 are connected in parallel to each other and to p-channel transistor 30. In the negative voltage portion of inverter circuitry 20, n-channel transistor 36 is connected in series with n-channel transistor 28, the gate of transistor 36 is connected to the V CC power supply, and transistors 28 and 36 are connected to the V SS power supply. N-channel transistors 38 and 40 are connected in parallel to each other and to n-channel transistor 36. The gates of transistors 30 and 36 may each be connected to input signal 22, rather than being connected to the V SS power supply and the V CC power supply, respectively. In the positive voltage portion of inverter circuitry 20, the gates of transistors 32 and 34 are driven by delay signals P1 and P2, respectively, which determine the amount of delay which will be introduced to the integrated circuit in the positive voltage direction. The resistive value of transistors 30, 32, and 34 are chosen such that when the value of delay signals P1 and P2 are appropriately chosen, the resultant resistive value in series with transistor 26 causes the desired delay in the circuit. For purposes of illustration, suppose the resistance of transistor 30 to be bigger than the resistance of transistor 32 which, in turn, is bigger than the resistance of transistor 34. When delay signals P1 and P2 are both equal to a logic low state, transistors 26, 30, 32, and 34 are turned on, and the resultant resistance in series with transistor 26 is equal to the resistive values of transistors 30, 32, and 34 in parallel with each other. Having P1 and P2 equal to a logic high state yields the largest resistance allowable for inverter circuitry 20: the resistive value of transistor 26 connected in series with the resistive value of transistor 30. The delay introduced to the switching circuitry of power supply V CC , therefore, is correspondingly large. It is possible to introduce a longer delay in inverter circuitry 20 by varying the values of delay signals P1 and P2. When delay signal P1 is equal to a logic high state and delay signal P2 is equal to a logic low state, transistor 32, having medium resistive value, is turned off, but transistors 30 and 34 are still on. The resultant resistance in series with transistor 26 is then the resistance of transistor 30 in parallel with the resistance of transistor 34. This resultant resistance is greater than the resistance formed when both P1 and P2 are at a logic low state as described above, and thus produces more delay. Therefore, by setting only delay signal P1 to a logic high state, it is possible to generate a longer delay than when both delay signals P1 and P2 are at a logic low state. Yet another delay time may be achieved by setting delay signal P2 to a logic high state, such that transistor 34 is turned off, and setting delay signal P1 to a logic low state. The resultant resistance in series with transistor 26 is equal to the value of the resistance of transistor 30 in parallel with the resistance of transistor 32. As described, delay signals P1 and P2 may be set to the appropriate logic levels to produce a desired delay in the positive voltage direction, i.e. in the V CC direction. Delay signals N1 and N2 have synonymous uses, but for the negative voltage direction, i.e. in the V SS direction. Therefore, delay signals N1 and N2 may each be set to a logic high state or a logic low state to produce desired delays in the negative going voltage direction. In this case, the desired delay would be produced by manipulating the resistances of transistors 36, 38, and 40 such that a desired resultant resistance in series with n-channel transistor 28 results. The function of delay signals P1, P2, N1, and N2 have been described above. These delay signals may be used to obtain global delay throughout an entire integrated circuit or circuit blocks of an integrated circuit. In other words, inverter circuitry 20 of Figure 2 may be duplicated in a plurality of circuit blocks of an integrated circuit sharing global delay signals such that by simply manipulating four global delay signals, P1, P2, N1, and N2, a desired delay time may be obtained throughout an integrated circuit device. Additionally, more delay times may be obtained by adding additional transistors in parallel with transistors 32 and 34. This is a highly desirable feature of the present invention in that it allows for a single fuse to be blown, a single test pad to be forced to a given logic level, or a test mode to be entered in order to set a delay signal to the desired logic state throughout an entire chip. Referring to Figure 3 , inverter circuitry 50 of a second type commonly used in switching circuitry of integrated circuits according to the prior art, is shown. Input signal 52 is inverted by inverter circuitry 50 which generates output signal 54, which is comprised of three series p-channel transistors 56, 58, and 60 and three series n-channel transistors 62, 64, and 66. Transistors 56, 58, and 60 are connected in series to V CC , a high voltage state, and transistors 62, 64, and 66 are connected in series to V SS , a low voltage state, as shown in Figure 3. The amount of delay in the circuit is determined by the resistive value of the transistors. For instance, assuming delay in the positive voltage direction, or V CC , the delay is determined by the resistive value of transistors 56, 58, and 60 connected in series. Likewise, for delay in the negative voltage director or V SS , the delay is determined by the resistive value of transistors 62, 64, and 66 connected in series. Figure 4 shows inverter circuitry 70 which allows for the impedance of power supplies V CC and V SS , when utilized by inverter circuitry 70, to be modified, according to a preferred embodiment of the invention. Inverter circuitry 70 is similar in function to inverter circuitry 50 of Figure 3, except that additional circuitry makes it possible to introduce selectable delay. Inverter circuitry 70 inverts input signal 72 to produce output signal 74. P-channel transistors 76, 78, and 80 are connected in series with each other and connected to V CC , as shown. Similarly, n-channel transistors 82, 84, and 86 are connected in series with each other and connected to V SS . In the positive voltage direction, transistors 88 and 90, corresponding to delay signals P1 and P2, respectively, allow for selectively choosing the positive voltage delay to be introduced to circuitry 70. Setting delay signals P1 and P2 equal to a logic low state causes transistors 76, 78, 80, 88 and 90 to be on, producing the smallest delay possible in a positive voltage direction. This minimum delay is possible because setting P1 to a logic low state causes the resistive values of transistors 76 and 88 to be connected in parallel which forms a first parallel resistive element 96, setting P2 to a logic low state causes the resistive value of transistor 90 to be connected in parallel with the series connection of the resistive value of transistor 78 and the first parallel resistive element which forms a second parallel resistive element 97. Therefore, the resultant resistance seen in a positive voltage direction is equal to the resistive value of transistor 80 connected in series with the resistive value of the second parallel resistive element 97. The width to length ratio of transistors 88 and 90 may be two to four times larger than the width to length ratio of transistors 76, 78, and 80, such that the resultant resistance is dominated by the resistive value of transistors 76, 78, and 80 rather than the resistive value of transistors 88 and 90. The largest positive voltage delay possible for circuitry 70 of Figure 4 may be realized by setting delay signals P1 and P2 to a logic high state. When delay signals P1 and P2 are both equal to a logic high state, transistors 88 and 90 are turned off, leaving only transistors 76, 78 and 80 on. Therefore, the delay is determined by the resistive value of transistor 76 in series with the resistive value of transistor 78 and the resistive value of transistor 80. Medium delay times may be introduced by setting delay signal P1 to a logic high level and delay signal P2 to a logic low level, or vice versa, in much the same way as described in Figure 2. If even greater delay resolution in the positive voltage direction is desired, more series transistors may be connected in series to transistors 76, 78, and 80. Delays in the negative voltage direction may be obtained by appropriately setting the logic state of delay signals N1 and N2. Values of delay signals N1 and N2 determine how the resistive values of transistors 82, 84, 86, 92, and 94 will affect the delay time. Setting delay signals N1 and N2 to a logic high state turns on transistors 92 and 94, and the resultant resistive value will be relatively small, causing a relatively small delay time. The resistive value of transistor 86 connected in parallel with the resistive value of transistor 92 defines a third parallel resistive element 98, and the resistive value of transistor 94 connected in parallel with the series connection of transistor 84 and the third parallel resistive element 98 defines a fourth parallel resistive element 99. In this case, the resultant resistive value in the negative voltage direction will be equal to the resistive value of transistor 82 connected in series with the fourth parallel resistive element 99, as defined above. The largest delay may be obtained by setting delay signals N1 and N2 each to a logic low state such that the resultant resistive value is equal to the resistive values of transistors 82, 84, and 86 connected in series. The width to length ratio of transistors 92 and 94 may be two to four times larger than the width to length ratio of transistors 82, 84, and 86, such that the resultant resistance is dominated by the resistive value of transistors 82, 84, and 86 rather than the resistive value of transistors 92 and 94. The function of delay signals P1, P2, N1, and N2 shown in Figure 4 has been described above. According to the present invention, these delay signals may be used to obtain global delay throughout an entire integrated circuit or circuit blocks of an integrated circuit. In other words, inverter circuitry 70 of Figure 4 may be duplicated in a plurality of circuit blocks of an integrated circuit sharing global delay signals such that by simply manipulating four global delay signals, P1, P2, N1, and N2, a desired delay time may be obtained throughout an integrated circuit device. This is a highly desirable feature of the present invention in that it allows for a single fuse to be blown, a single test pad to be forced to a given logic level, or a test mode to be entered in order to set a delay signal to the desired logic state. The introduction of global delay times may be used to control delays for sense amplifiers and edge transition detection (ETD) pulses. It often becomes important to be able to adjust the ETD pulse width to allow for thorough device characterization and device optimization. ETD circuitry 100, according to the prior art, is shown in Figure 5 . ETD circuitry 100 operates such that when any one of n-channel transistors 102 or 104 conducts because its gate signal, 103 or 105 respectively, is equal to a logic high state, node 114 is pulled to a logic low state (V SS ), and the output signal 112 of inverter 110 also goes to a logic low state. P-channel transistor 106 is turned on, and node 114, which feeds the input of inverter 108, is correspondingly pulled to a logic high state. When node 114 is pulled up to a sufficiently high logic state, output signal 112 goes to a logic high state and transistor 106 turns off. The present invention as in Figure 4 may be used to adjust ETD pulse width. ETD circuitry 120 of Figure 6 employs an adjustable delay block 130, to determine ETD pulse width, according to a third embodiment of the present invention. Adjustable delay block 130 is synonymous with the inverter circuitry shown in either Figure 2 or Figure 4, but may be replaced by circuitry which performs other logic functions, such as a NAND or a NOR logic function. As in Figure 2 or Figure 4, adjustable delay block 130 has multiple delay signals, such as P1 and P2 shown in Figures 2 and 4 which may be accordingly set to produce desired delay times in the positive voltage direction. A plurality of n-channel transistors 122, 124 are connected in parallel to one another and connected to V SS . Of course, additional n-channel transistors may be placed in parallel to transistors 122 and 124. When at least one of transistors 122 or 124 conducts because its gate signal, 123 or 125 respectively, is equal to a logic high state, node 136 is pulled to a logic low state (V SS ), and the output signal 134 of inverter 128 also goes to a logic high state. After some delay time determined by adjustable delay block 130, p-channel transistor 126 is turned on, causing node 136, which feeds the input signal of inverter 128, to be pulled up to a logic high state. When node 136 is pulled up to a sufficiently high logic state, output signal 134 goes to a logic low state, the output signal 131 of adjustable delay block 130 goes to a logic high state, and transistor 126 turns off. Referring now to Figure 7 , a circuit is shown for introducing delay to ETD circuitry 140. Figure 7 is similar to ETD circuitry 120 of Figure 6, with a major difference being the use of pass gates 152 and 154 to introduce delay to ETD circuitry 140 in Figure 7. Delay signal 158 determines the resistive value connected in series with the gate of p-channel transistor 146, and thus the delay which will be introduced. A plurality of n-channel transistors 142, 144, etc. are connected in parallel to one another and tied to V SS . Of course, additional n-channel transistors may be placed in parallel to transistors 142 and 144. When at least one of transistors 142 or 144 conducts because its gate signal, 143 or 145 respectively, is equal to a logic high state, node 164 is pulled to a logic low state (V SS ), and the output signal 162 of inverter 150 also goes to a logic low state. After some delay dependent upon the resistive value of pass gate 152 in parallel with the resistive value of pass gate 154, p-channel transistor 146 turns on, if delay signal 158 is equal to a logic high state. As a result of transistor 146 being turned on, node 164, which feeds the input signal of inverter 148, is pulled up to a logic high state. When node 164 is pulled up to a sufficiently high logic state, transistor 146 will turn off. Delay signal 158 determines how the resistive values of pass gate 152 and pass gate 154 will affect the delay introduced to ETD circuitry 140. When delay signal 158 is equal to a logic high state, pass gate 154 is turned on, meaning that the resulting resistance in series with the gate of p-channel transistor 146 is equal to the resistive value of pass gate 152 in parallel with the resistive value of pass gate 154. Since the resistance is lowered by having both pass gates 152 and 154 on and connected in parallel to each other, the resultant delay is correspondingly shorter than if pass gate 154 were not on. It then follows that when delay signal 158 is equal to a logic low state, pass gate 154 is turned off, and thus the resistance in series with the gate of p-channel transistor 146 is equal to the resistive value of pass gate 152. Since the resultant resistance is higher when delay signal 158 is equal to a logic low state, the delay introduced to ETD circuitry 40 is also higher. The function of delay signals P1 and P2 of Figure 6 and delay signal 158 of Figure 7 has been described above. These delay signals may be used to obtain global delay or to control pulse widths throughout an entire integrated circuit or circuit blocks of an integrated circuit. In other words, the ETD circuitry 120 of Figure 6 or the ETD circuitry 140 of Figure 7 may be duplicated in a plurality of circuit blocks of an integrated circuit such that by simply manipulating the appropriate global delay signals, a desired delay time may be obtained throughout an integrated circuit device. This is a highly desirable feature of the present invention in that it allows for a single fuse to be blown, a single test pad to be forced to a given logic level, or a test mode to be entered in order for a delay signal to be set to the desired logic state, and all delay elements connected to the delay signal are affected. One embodiment of the present invention for globally introducing adjustable delay or pulse width throughout an integrated circuit has been shown. That embodiment allows for blowing a single fuse, forcing a single test pad, or entering into a test mode to set a delay signal to the appropriate logic level. As previously mentioned, it is important to be able to blow a fuse to introduce desired delays to integrated circuitry for production devices, just as it is important to force a test pad to a given level during wafer testing and to enter a test mode during testing of the device. Additionally, the invention does not affect normal operation of an integrated circuit in which it is used and is layout efficient.

Claims: Circuitry for selectively introducing delays to a signal in an integrated circuit, comprising: a plurality of logic elements, each logic element having an input terminal (72) for said signal, an output terminal (74) for a respective output signal, a first transistor (80) having a resistive value, a second transistor (78) having a resistive value, a third transistor (76) having a resistive value, a fourth transistor (88) having a resistive value, a fifth transistor (90) having a resistive value, and a sixth transistor (82), wherein the sixth transistor (82), the first transistor (80), the second transistor (78), and the third transistor (76) are connected in series, the third transistor (76) is connected in parallel to the fourth transistor (88) to define a first parallel resistive element (96;76,88) having a resistive value, and the fifth transistor (90) is connected in parallel to the series connection of the first parallel resistive element (96;76,88) and the second transistor (78) to define a second parallel resistive element (97;76,88,78,90) having a resistive value, wherein said signal drives the gates of the first transistor (80), the second transistor (78), the third transistor (76) and sixth transistor (82) and the output signal is produced at a common node between the first transistor (80) and the sixth transistor (82), wherein the plurality of logic elements are located in a plurality of distributed circuit blocks throughout the integrated circuit; a first supply voltage connected in series with the third transistor (76); a second supply voltage coupled to the sixth transistor (82); and a first global delay signal (P1) connected to the gate of the fourth transistor (88) and a second global delay signal (P2) connected to the gate of the fifth transistor (90), such that when the first global delay signal and the second global delay signal are equal to a first logic state, the fourth transistor and the fifth transistor conduct and a first resultant resistance in series with the resistive value of the first transistor (80) defines a first delay time, wherein the first resultant resistance is the resistive value of the second parallel resistive element (76,88,78,90); and when the first global delay signal and the second global delay signal are equal to a second logic state, the fourth transistor (88) and the fifth transistor (90) do not conduct and a second resultant resistance in series with the resistive value of the first transistor (80) defines a second delay time, wherein the second resultant resistance is the resistive value of the second transistor (78) in series with the resistive value of the third transistor (76); and when the first global delay signal is equal to the first logic state and the second global delay signal is equal to the second logic state, the fourth transistor (88) conducts and a third resultant resistance in series with the resistive value of the first transistor (80) defines a third delay time, wherein the third resultant resistance is the resistive value of the first parallel resistive element (76,88) in series with the resistive value of the second transistor (78); and when the first global delay signal is equal to the second logic state and the second global delay signal is equal to the first logic state, the fifth transistor (90) conducts and a fourth resultant resistance in series with the resistive value of the first transistor (80) defines a fourth delay time, wherein the fourth resultant resistance is the resistive value of the fifth transistor (90) connected in parallel to the series connection of the resistive value of the second transistor (78) and the resistive value of the third transistor (76). Circuitry as claimed in claim 1 further comprising seventh to tenth transistors (84,86,92,94), each of said seventh to tenth transistors having a respective resistive value, wherein said seventh (84) and eighth (86) transistors are connected in series and have gates driven by said signal, said ninth transistor (92) is connected in parallel with said eighth transistor to define a third parallel resistive element (98) and the tenth transistor (94) is connected in parallel to the series connection of the third parallel resistive element (98) and the seventh transistor (84) to define a fourth parallel resistive element (99) having a resistive value, wherein the signal drives the gates of the seventh transistor (84) and the eighth transistor (86); wherein said second supply voltage is connected in series with the eighth transistor; and a third (N1) and a fourth (N2) global delay signal are connected respectively to the gate of the ninth transistor (92) and the tenth transistor (94). Circuitry as claimed in claim 1 or claim 2 wherein said first to fifth transistors are p-channel transistors (76,78,80,88,90) and the sixth transistor (82) is an n-channel transistor. Circuitry according to claim 2 wherein said seventh to ninth transistors (84,86,88,94) are n-channel transistors. Circuitry as claimed in claim 1 wherein the logic elements are located in a plurality of distributed circuit blocks throughout the integrated circuit.

Inventors: MCCLURE DAVID CHARLES, MCCLURE, DAVID CHARLES
Applicants: SGS THOMSON MICROELECTRONICS, STMICROELECTRONICS, INC.
Application year: 1994
Main classification: H03K5
IPC codes: G01R31, H01L27, H03K5, H01L21